(Reuters) - Financial markets have fretted about the fiscal situation in Washington, with deadlines looming in late September and early October to keep the U.S. government open and raise the debt ceiling.

As Congress returned on Tuesday, aiding those affected by Hurricane Harvey and responding to President Donald Trump’s decision to scrap a program shielding immigrants brought to the U.S. illegally as children from deportation added to their already-packed agenda.

Here is what you need to know about fiscal and other politically connected issues in play this month:

WHAT IS THE DEBT CEILING?

The “debt ceiling” is a legal cap on how much money the U.S. government can borrow through bonds and other debt issued by the U.S. Treasury. It now stands at about $19.8 trillion, very close to the actual national debt.

Once the ceiling is hit, Congress must raise it. If it does not, the government cannot keep borrowing. This matters because the government spends more than it collects in taxes and Washington continually borrows money to cover its bills. Legislation to raise the debt limit will need to be adopted by early October at the very latest.

The Treasury, however, has said that Congress must increase the debt ceiling by Sept. 29. Since March, the Treasury has been using “extraordinary measures” to stave off hitting the ceiling and likely could be stave off default a few more weeks by using such extraordinary measures, analysts have said.

WHAT IF THE CEILING IS NOT RAISED?

If the debt ceiling is not raised and the government can no longer pay its bills, default would result, along with a likely downgrade in the U.S. credit rating.

This has never happened before, but there have been some close calls. A standoff in August 2011 cost the country its top-notch bond rating from the credit ratings agency Standard & Poor’s and caused the most jarring two weeks in financial markets since the 2007-2009 global financial crisis.

WHAT IS A GOVERNMENT SHUTDOWN?

Congress is supposed to pass annual spending bills around the end of the federal fiscal year on Sept. 30 to fund most of the U.S. government, but disagreements often prevent this.

When that happens, lawmakers usually pass a temporary bill extending current spending levels with no changes for a short period. It is expected this will happen again at the end of this month, likely extending current funding levels into December.

WHAT IF CONGRESS FAILS?

Congress returned from its long summer recess having only about 12 working days to pass a spending measure, probably a short-term patch, known as a continuing resolution, to keep the government open.

If even a short-term patch cannot be agreed upon in Congress, or if the president decides to veto either a long-term or short-term spending measure, the government would shut down.

The last time the government shut down was in October 2013 for about two weeks. In the 1990s, 1980s and 1970s, there were 17 shutdowns. Shutdowns hurt federal workers, rattle markets and shake confidence in the United States abroad, but they have done little lasting economic damage.

HOW ARE DEBT CEILING, SPENDING MEASURE RELATED?

The two move on separate tracks but could get tangled together as some analysts have said that Congress may try to tackle both issues at the same time, perhaps in a single piece of legislation.

WHAT ABOUT HURRICANE HARVEY?

Passing an initial tranche of federal aid, expected to be about $8 billion, for individuals affected by Hurricane Harvey now tops the congressional agenda.

Senate Majority Leader Mitch McConnell said on Tuesday that raising the debt ceiling, keeping the government open and passing an assistance package for Harvey victims were three “critically important things” that Congress needed to get done quickly.

There has been some speculation that Harvey aid could be tied to a short-term continuing resolution or a measure raising the debt ceiling, or that all three could be linked in some way.

WHAT ABOUT THE FLOOD INSURANCE PROGRAM?

Separate from Harvey assistance is the National Flood Insurance Program, operated by the Federal Emergency Management Agency, which expires on Sept. 30.

The program provides insurance policies for homeowners that cover up to $350,000 in damages. Congressional disagreements over changes to the program, which operates in debt to the Treasury, have complicated plans to restructure the program in the past.

It is not yet clear how Harvey, which will generate additional claims made to the cash-strapped program, will affect the timing and scope of its renewal.

WHAT ABOUT THE DREAMERS?

Trump on Tuesday scrapped an Obama-era program known as Deferred Action for Childhood Arrivals, DACA, that protects from deportation those who were brought illegally to the U.S. when they were children.

Trump has delayed implementation of his decision for six months, saying it would give Congress time to figure out a legislative fix that would affect almost 800,000 young people who participate in the program, known as Dreamers.

It is not yet clear whether Congress will work to quickly pass a measure affecting the Dreamers or pursue a larger-scale overhaul of the immigration system.

WHAT WILL HAPPEN TO OBAMACARE?

The chances that the Republican-led Congress will continue making a priority of repealing and replacing former President Barack Obama’s signature healthcare law are slim after previous efforts ended in defeat. But important Obamacare-related deadlines loom.

The Trump administration has been making Obamacare payments to insurers that reduce out-of-pocket costs for lower-income Americans on a month-to-month basis and must decide before the end of September whether to make October’s payment. If it does not, Congress could step in to approve the payments with additional legislation.

Related to but not a part of Obamacare is the Children’s Health Insurance Program, CHIP, a federal insurance program for millions of lower-income children and pregnant women that requires congressional reauthorization by Sept. 30.

CHIP reauthorization is not typically contentious as the program receives bipartisan support. But lobbyists and industry officials have said any healthcare-related legislation has become more complicated in the wake of the Obamacare repeal-and-replace failure.

WHEN WILL CONGRESS TAKE UP TAX REFORM?

Trump kicked off the new legislative session’s tax reform effort on Tuesday by inviting key Republican leaders to the White House. An outline of the plan being developed could come as early as the week of Sept. 11.

Senate Republicans want to use a special process known as budget reconciliation to pass tax reform, because then they would only have to find a simple majority of 51 senators to pass it instead of the 60 votes that typically required in the 100-seat Senate, where Republicans hold 52 seats.

But to use this process, Republicans will first have to pass a 2018 budget resolution, which they so far have been unable to do.